Normalize url

This patch implements the net (after conversation) functionality of the following two bullets on the Walker Project Plan

* Ensure we normalize URLs (NOTE: some of these may be a non-issue, just want to check them):
* Configurable parameter exclusion (case insensitive) with defaults: jsessionid, phpsessid, aspsessionid

If one takes a look at the purell source code: it modifies it's url argument in place. So this code does the right thing [just for the record I don't like the API for this library, but ...]. Obviously, that fact isn't as clear as it needs to be. What about a comment to that extent above the call to NormalizeURL?